1. Field of the Invention
This invention relates to methods for ensuring purity of potable water supplies. More specifically, this invention relates to methods for avoiding stagnation of such water supplies in storage tanks.
2. Description of the Related Art
Stagnant water is a leading cause of the deterioration of drinking water stored in water storage tanks. When a large capacity tank is underutilized, differential thermal conditions in the tank can cause the contents to stratify in thermoclines, where warmer layers of water meet cooler layers. As is well known in the art, the accumulation and growth of algae, protozoan pathogens such as cryptosporidium and other undesirable organisms is favored at such thermoclines.
If, as is often the case, a tank with stratified contents is both filled and emptied from a limited portion of the tank, water supplied by the tank will be from recently filled, fresher strata, while the remaining strata in the tank may age and harbor increasing microbial populations, becoming stagnant.
In many public water systems, water is disinfected before it enters the storage tank to ensure that potentially dangerous microbes are killed before the water enters the distribution system. Because residual disinfectant remains in the water after treatment, disinfectant agents such as chlorine, chloramines or chlorine dioxide provide further protection from microbial reproduction after water enters the distribution system. The efficacy of such residual disinfectants diminishes with time, however. When disinfected water is allowed to stratify in storage tanks, older layers of water may lose disinfectant protection altogether, leading to the possibility that such portions of the tank become stagnant despite disinfectant treatment of water prior to transport to the tank.
What is needed is a method of preventing or remediating stratification of water in storage tanks. As will be understood by those in the art, stratification can be obviated by sufficient vertical mixing of water in the tank.
A number of means for mixing liquids are available to de-stratify stored water. A mechanical mixer, comprised of a screw or blade that is turned by a motor, is commonly employed to mix various liquids. Mechanical mixers, however, are subject to a number of shortcomings for mixing drinking water in storage tanks.
Mixing the strata in a typical large water storage tank with a mechanical mixer requires a large amount of energy relative to the amount of water that is actually mixed. Further, agitation of the water in the tank by mechanical mixers can disturb sediment settled in the bottom of the tank, resulting in suspended sediment degrading the aesthetics of the water for drinking. Further still, mechanical mixers are often inefficient, mixing some but not all strata in a storage tank. In addition, acquisition costs can be high for a mechanical mixer having sufficient capacity to mix all the strata in a large storage tank. Yet further, costs are high to retrofit an existing water storage tank with a mechanical mixer, retrofitting further often entailing a need to drain the tank or otherwise temporarily remove the tank from the water distribution system. What is needed are more economical and efficient means of mixing water to eliminate stratification with minimal disturbance to sediment in the tank. What is needed further is such means that can be retrofitted to a water storage tank operation economically and without a need to take the water tank off-line.
For economy, it is further desirable that the mixer that is used to obviate stratification be engaged only when needed, i.e. only when stratification is taking place. Accordingly, it is desirable to have a means for determining when mixing is needed and for engaging the mixer only at such times.
It is further desirable that the mixer system be easy to install and easy to operate.